How To Destroy A Black Hole

Astrophysicists think they know how to destroy a black hole. The puzzle is what such destruction would leave behind

The idea of a body so massive that its escape velocity exceeds the speed of light dates back to the English geologist John Michell who first considered it in 1783. In his scenario, a beam of light would travel away from the massive body until it reached a certain height and then returned to the surface.

Modern thinking about black holes is somewhat different, not least because special relativity tells us that the speed of light is a universal constant. The critical concept that physicists focus on today is the event horizon: a theoretical boundary in space through which light and other objects can pass in one direction but not in the other. Since light cannot escape, the event horizon is what makes a black hole black.

The event horizon is somewhat of a disappointment to many astrophysicists because the interesting physics, the stuff beyond the known laws of the universe, all occurs inside it and is therefore hidden from us.

What physicists would like, therefore, is way to get rid of the event horizon and expose the inner workings to proper scrutiny. Doing this would destroy the black hole but reveal something far more bizarre and exotic.

Today, Ted Jacobson at the University of Maryland and Thomas Sotiriou and the University of Cambridge explain how this might be done in an entertaining and remarkably accessible account of the challenge.

In general relativity, the mathematical condition for the existence of a black hole with an event horizon is simple. It is given by the following inequality: M^2 > (J/M)^2 + Q^2, where M is the mass of the black hole, J is its angular momentum and Q is its charge.

Getting rid of the event horizon is simply a question of increasing the angular momentum and/or charge of this object until the inequality is reversed. When that happens the event horizon disappears and the exotic object beneath emerges.

At first sight, that seems straightforward. The inequality suggests that to destroy a black hole, all you need to do is to feed it angular momentum and charge.

But that hides a multitude of problems. For a start, things with angular momentum and charge also tend to have mass. And in any case, the equation above describes a steady state. Feeding a black hole creates a dynamic state and there is no guarantee that the object will settle back into a steady state again without shedding the angular momentum and charge that it has been fed.

In fact, the calculations are so fiendish that they have defied all attempts to tame them. “At present nobody knows what it would do,” say Jacobson and Sotiriou.

What would a black hole without its event horizon reveal? That’s where physics turns philosophical. The mathematics here indicates that spacetime becomes infinitely curved, creating what astrophysicists call a singularity.

To any ordinary physicist, a singularity is an indication that a theory has broken down and some new theory is needed to describe what is going on. It is a matter of principle that singularities are mathematical objects, not physical ones and that any ‘hole’ they suggest exists not in the fabric of the Universe but in our understanding of it.

Astrophysicists are different. They have such extraordinary faith in their theories that they believe singularities actually exist inside black holes. The likes of Roger Penrose and Stephen Hawking have even proved that singularities are inevitable in gravitational collapse.

For them, removing the event horizon around a black hole raises the exciting prospect of revealing a singularity in all its naked glory. When that happens, we will be able to gaze at infinity.

That seems bizarre.

Destroying a black hole in this way is bound to reveal new physics. But whatever this might be is bound to remain well hidden until we have a theory that better describes what goes on in such extremes. Or until we spot one of these objects somewhere in the night sky.